Daurichromenic acid (1a) was isolated from the leaves and twigs of Rhododendron dauricum, a plant that is distributed in the northern part of China, eastern part of Siberia, and Hokkaido. See, e.g., Kashiwada, Y. et al., Tetrahedron, 57, 1559 (2001); Jpn. Kokai Tokko Koho, JP 82-28, 080, 1982. It belongs to the family of chromene natural products. Two novel chromane derivatives rhododaurichromanic acids A (5a) and B (6b) were also isolated from the same plant:

Synthesis of 2H-benzopyrans (chrom-3-enes) has been the subject of many investigations. Dotz, K. H. Pure & Appl. Chem. 1983, 55, 1689 and references cited therein; (b) Henry, G. E.; Jacobs, H. Tetrahedron 2001, 57, 5335; (c) Chang, S. et al., J Org. Chem. 1998, 63, 864; (d) Saimoto, H. et al., J Org. Chem. 1996, 61, 6768; (e) North, J. T. et al., J Org. Chem. 1995, 60, 3397; (f) Gabbutt, C. D. et al., Tetrahedron 1994, 50, 2507; (g) Cruz-Almanza, R. et al., Heterocycles 1994, 37, 759; (h) Rao, U. et al., Tetrahedron Lett. 1983, 24, 5023; (i) Sartori, G. et al., J. Org. Chem. 1979, 44, 803. The reaction developed by Shigemasa appeared to be quite promising for the synthesis of this class of natural products. Saimoto, H. et al., J Org. Chem., 1996 61, 6768. Unfortunately, the reaction between 16 and 3a is extremely slow under Shigemasa's conditions (see FIG. 1). The mixture gave only 15% yield of the desired product 4a (B=Et) after heating at reflux for four days (see Table 1, hereinbelow, entry 1). The yield was improved to 32% when the mixture was heated at 90° C. in a sealed tube for one day (Table 1, entry 2). However, the reaction stopped, and the yield is not improved even with the addition of excess of aldehyde 3a and with longer heating time.
Therefore, a need exists for effective methods to prepare daurichromenic acid and analogs thereof.